The invention relates to a process for separating C.sub.2+ or C.sub.3+ hydrocarbons, or C.sub.4 hydrocarbons, from a gaseous mixture containing these components and lighter constituents by cooling and rectification.
Partial condensation represents a simple separating process wherein a gaseous stream to be fractionated is separated into two fractions of differing composition merely by lowering the temperature to below the dew point and by subsequent phase separation in a separator. This procedure constitutes a poorly selective separation with only one equilibrium stage wherein higher-boiling components are enriched in the condensate and lower-boiling constituents are enriched in the gaseous phase. On account of the low separating selectivity, such processes are utilized essentially as preliminary separating stages upstream of a larger fractionating unit, for example upstream of a rectifying column.
A more selective separation, however, at the cost of increased expenditure, is possible, for example, when using a dephlegmator or, respectively, reflux condenser. In a reflux condenser, the upwardly flowing gaseous mixture is cooled by indirect heat exchange with a cooling medium, preferably the cold, uncondensed gas; with increasing cooling action, an increasing number of components of the gaseous mixture is condensed and drops downwards in the opposite direction to the gaseous stream. The downwardly dropping liquid enters into heat exchange and mass transfer with the rising gaseous mixture so that a rectifying gas separation takes place within the reflux condenser. The heavy component to be separated can be withdrawn from the bottom of the reflux condenser whereas the cold gas, freed extensively of the heavy component and exiting at the top from the reflux condenser, can be recycled, for example after a cold-producing expansion, countercurrently to the gaseous mixture to be fractionated, as a coolant to the reflux condenser where it is heated approximately again to the initial temperature by indirect heat exchange before being passed on to further usage. Heat exchangers are ordinarily installed in the reflux condenser for cooling the gaseous mixture; these heat exchangers, on account of their specific design, for example as wound heat exchangers or plate-type heat exchangers, effect an intensive contact between the upwardly flowing gaseous mixture and the downwardly dropping liquid.
EP-A 126,309, for example, discloses such a process wherein a gaseous mixture containing at most 10% C.sub.4 hydrocarbons and at least 65% methane and lighter components is fractionated with the use of a dephlegmator or reflux condenser.
The use of a reflux condenser instead of a usual rectifying column offers advantages, in particular, under the viewpoints of thermodynamics since the return flow required for rectification of the gaseous mixture is produced within the reflux condenser proper at a sliding temperature, whereas in case of a rectifying column the entire return flow must be made available at maximally low temperature and must be introduced into the head of the column. The fact that the return flow is formed with sliding temperature from the gaseous mixture proper can, however, also be an obstacle to the use of a reflux condenser in special cases. If, for instance, the gaseous mixture to be fractionated consists essentially of components having widely spaced-apart boiling points, then a large portion of the higher-boiling component condenses already shortly below the dew point in the lower region of the reflux condenser. However, in case this component is to be separated in high yield, temperatures must be set to far below the dew point, for example up to 100.degree. C. below the dew point. Such an irregular course of the condensation is not advantageous because with dropping temperatures, there is an increasingly smaller quantity of newly formed condensate, i.e. there is an increasingly smaller return flow produced for the upper region of the reflux condenser. As a consequence, the rectification becomes ineffective, and the entire reflux condensation becomes questionable